Some questions about __init__.py

enviroment: python3.8

directory:

test\
    module1\
           __init__.py
           mod1.py
    module2\
           mod2.py
    main.py

1. I can import a directory without __init__.py , why?

I want to check how exactly init.py work, so I import the module1 and module2 in main.py

# test\main.py
from module1 import mod1
from module2 import mod2

In my expectation, line2 should occur error, because I didn't create __init__.py in module2

But both of them can be imported.

Now, I'm confused about how exactly __init__.py work

2. I can't import those package which is imported in __init__.py

I try to import mod1.py in main.py in this way:

# test\main.py
import module1
module.mod1.mod1_print() # mod1_print() is a function which is defined in mod1.py

So I import mod1.py in test\module1\__init__.py :

# test\module1\__init__.py
import mod1

But when I execute main.py I got this error:

D:\>"C:/Program Files/Python38/python.exe" d:/test/main.py
Traceback (most recent call last):
  File "d:/test/main.py", line 2, in <module>
    import module1
  File "d:\test\module1\__init__.py", line 1, in <module>
    import mod1
ModuleNotFoundError: No module named 'mod1'

My question is:

If I want to import mod_print() in this form: module.mod1.mod1_print() , what should I do?